Conventionally, a torque limiter, which is described in the publication of JP 63-30527 B or the like, plays the role of preventing, e.g. in steel rolling mills or the like, such expensive machines (parts) as universal joints (UJ) from malfunctioning due to overloads applied thereto. The torque limiter, while burdened with a load within a specified range for transmitting torque to a shaft member or a cylinder member, contracts the diameter of the inner circumferential surface of the cylinder member by a hydraulic passage that has expanded by feed of oil for use of oil pressure expansion so that the outer circumferential surface of a shaft member internally fitted to the inner circumferential surface of the cylinder member is tightened by the inner circumferential surface of the cylinder member, thus making the shaft member and the cylinder member frictionally coupled to each other to fulfill torque transmission.
Also, when a load over a specified value is applied to the shaft member or the cylinder member so that the inner circumferential surface of the cylinder member is rotated relative to the outer circumferential surface of the shaft member with the result that the shaft member is changed in its circumferential position relative to the cylinder member, the oil for oil pressure expansion is bled out from within the hydraulic passage of the cylinder member to release the inner circumferential surface of the cylinder member from pressing against the outer circumferential surface of the shaft member, thereby undoing the frictional coupling between the shaft member and the cylinder member to cut off the torque transmission. The design release torque (a torque at which a torque limiter starts idling) of the torque limiter is set between a fatigue strength (a strength which, even if a torque of the same magnitude is applied to a machine such a large number of times as 107 or so, does not cause break of the machine) of a machine that is to be prevented from breakage by the torque limiter being connected thereto, and a static strength (a strength which, even if a torque of the same magnitude is applied to a machine only once, causes the machine to break) of the machine. In this way, the torque limiter enhances the torque to be transmitted in magnitude, and moreover, with a load of a specified value or more applied to the shaft member or the cylinder member, protects expensive machines connected to the torque limiter or prevents personal injuries.
In recent years, from demands for lessened placement space and the like, there has been a tendency toward smaller sizes of the machines (e.g., universal joints), which involves a tendency toward lowered static strengths of those machines.
FIG. 7 is a chart for explaining a problem occurring due to a decrease in the static strength of the machine. It is noted that in FIG. 7, a universal joint (UJ) is used as an example of the machine.
As shown in FIG. 7, when the UJ is reduced in diameter (reduced in size) with the static strength of the UJ decreased, a difference denoted by ‘a’ in FIG. 7 between the static strength and a set value of the release torque decreases down to a difference denoted by ‘b’ in FIG. 7. In this case, if the value of the release torque is subjected to a large variation and this variation is larger than ‘b’, then there is a possibility that the UJ may break. Therefore, the demand for a torque limiter having smaller variations of the release torque has been increasing.    Patent Document: JP 63-30527 B